1. Field of the Invention
The present invention relates generally to spectrophotometry, and, more particularly, concerns a method of compensating for error in the amount of indicator added to a sample in bleaching chemistry analyses.
2. Description of the Prior Art
Chemical analyzers such as, for example, spectrophotometers, automatic chemical analyzers and the like, for monitoring a wide variety of water parameters have long been known to the art and industry. Typically, such analyzers read a color change produced by the addition to the sample of a suitable reagent to indicate the concentration of the substance for which the test is designed. Color changes are efficiently detected by sensing transmitted light filtered to the particular color, or wavelength, involved in the given test. By well known analytical methods, the parameter concentration may be ascertained. Single beam and double beam analyzers are the apparatus commonly used for carrying out such methods.
Chemical analyzers have been thus very effective and successful where "conventional" chemistries, have been used in a given test. In such tests, the presence and concentration of a parameter of interest are indicated by an increase in absorbance at the selected wavelength. The indicator is typically added to the sample in an amount in excess of the amount needed to react with all parameter expected to be found in the sample so that the amount of parameter in the sample can be accurately ascertained. Neither the parameter of interest nor the indicator, individually, contribute to the absorbance read by the instrument at the wavelength which is being measured. The sample parameter readily combines with the indicator reagent to form a color (i.e., absorbance change) which is measured by the instrument. Since the indicator is added in excess, any color change in the sample is attributed only to the sample parameter. By known methods, the parameter concentration may then be determined. Obviously, the exact amount of indicator added to the sample is not critical to the analysis. More particularly, when conventional chemistries are used in conjunction with chemical analyzers to conduct the desired test, error in the amount of indicator added to, that is, metered into, successive samples does not affect the accuracy of the analysis so long as the indicator is added in excess.
On the other hand, in "bleaching" chemistries, i.e., those in which the presence and concentration of a parameter of interest is indicated by a decrease in absorbance at the wavelength being measured, the quantity of indicator added to the sample is critical to the analysis. In this type of analysis an indicator having a strong color, that is, strong absorbance, which the instrument measures at the measuring wavelength, is added to the sample. The indicator combines with the parameter to form a colorless compound at the measuring wavelength. As a result, the color being measured by the instrument is reduced. By known analytical methods, the color change may be used to determine the amount of parameter in the sample.
Obviously, since color reduction is used to determine the amount of parameter present in the sample, regardless of whether the analysis is done with a chemical analyzer, or manually, for accurate and precise analyses it is critical that the exact amount of indicator added to the sample be known. Any error in the amount of indicator added to the sample will cause error in the color measured so that such indicator error will cause an erroneous analytical result. Thus, for example, if the amount of indicator in one analysis is lower than in another analysis, the color measured will be reduced in the former analysis in comparison to the latter one. As a result, it will appear that more bleaching has occurred in the former analysis, thus erroneously signalling an increase in the parameter concentration.
This problem is troublesome whenever bleaching chemistry analysis is used to monitor parameter concentration, and is particularly troublesome when automatic chemical analyzers are used to monitor, for example, process water effluent where successive water samples are continuously analyzed for the purpose of determining the parameter concentration. Any variation in the amount of reagent metered into (added to) successive samples will result in erroneous color change measurements from sample to sample thereby adversely effecting the accuracy and precision of the monitoring function. Moreover, neither single wavelength analyzers nor double beam analyzers can compensate for indicator variations in bleaching chemistry analyses, and particularly chemical analyzers, so as to eliminate the error caused by such variations.
One technique commonly used to compensate for sample turbidity and color uses single beam dual wavelength optics to conduct the given test. A beam of light is split so that a portion of the light is directed through a filter to a reference detector and another portion of the light is directed through a filter to a sample detector. The wavelength of the sample and reference filters are chosen on the basis of the parameter being tested and the likely interferences to be found in the sample, respectively. Thus, the light passing through the filter to the reference detector is attenuated by sample turbidity or color and the light passing through the filter to the sample detector is attenuated by the color produced by the reaction of parameter with indicator. Accordingly, the reference detector produces a reference signal related to sample turbidity or color, to which the signal from the sample detector can be compared. Signals from the two detectors are fed to an electrical system which finds the logarithm of the ratio between the sample and reference signals to produce a parameter reading for the sampled water.